In hark disk drives, the ramp loading (parking) of the pick up carrying arm, in case of an external interruption of the power supply to the hard disk drive, is made possible by exploiting the spindle motor as a generator. The voltage of the generator depends on the speed of rotation and the electrical constant of the motor. By rectifying the back electromotive forces (BEMF) induced in the phase windings of the motor, a rectified charge current is forced through a hold capacitor connected between the power supply leaves of the output drive bridge stage of the multiphase spindle motor and a voice coil motor for moving the pick up arm to power the latter for the time needed for safely parking the pick up carrying arm.
FIG. 1 depicts a typical power control device of a hard disk drive. The externally applied supply voltage VCV powers the output bridge stages of the spindle motor and the VCM motor vary according to the application. In desktop applications, it is generally 12V while in lap top applications it is generally 5V.
The power control device, besides integrating the control circuitry of the two motors, spindle and VCM, includes other functional blocks such as the voltage regulators, the power monitor, the serial interface and the ISO-Fet device. The ISO-Fet is an internal element that isolates the internal power supply node from the external supply VCV when the latter is interrupted.
The turning on of the ISO-Fet device is controlled by the POR signal that becomes active when all the internally monitored voltages are at a correct level, thus turning on the ISO-Fet. Therefore, if accidentally the external power supply voltage VCV is no longer present, the back electromotive forces that are induced in the windings of the rotating spindle motor are rectified by the intrinsic current recirculation diodes of the six power MOSFETs of the output bridge stage of the spindle motor. This maintains the inner power supply rail Vmotor at a level sufficient to power the control circuitry of the VCM motor to safely park the pick up carrying arm.
In many desk top applications (12V) and even more so in mobile applications (5V), the voltage obtained by rectifying through the intrinsic diodes of the integrated structures of the six power MOSFETs could be insufficient to provide a sufficient level of rectified voltage to ensure a correct functioning of the circuitry and of the VCM to safely park the pick ups.
To enhance the efficiency of recovery of the back electromotive force induced in the phase windings of the spindle motor, two approaches are commonly followed. They are synchronous rectifying of the BEMF of the spindle motor, and step-up of the spindle motor.
According to the first approach, the rectifying of the BEMF of the spindle motor takes place in an active fashion through the sequential turning on of two MOSFETs of the output bridge stage of the spindle motor synchronously with the three BEMFs that are sequentially reduced in the respective three phase windings of the three phase spindle motor depicted in FIG. 1.
According to the second approach, instead of the output bridge stage being kept in a high impedance (tristate) condition, it is continuously switched from a tristate condition to a condition of braking of the motor at a relatively high frequency, e.g., at 16 KHz or above, so as to be outside the acoustic band.
In this way, when the output stage is in a condition of braking, either with all the low side MOSFETs turned on or all the high side MOSFETs turned on, the windings of the spindle motor are in a short circuit condition. Therefore, all three BEMFs contribute to generate current in the motor windings. Thereafter, when the output stage is driven again to a tristate condition, the motor current generated during the phase of braking recirculates through the intrinsic diodes of the integrated structures of the six power MOSFETs, thus charging the hold capacitor C3 connected to the V motor node.
Referring now to the circuit diagram of FIG. 2, the step-up function of the spindle motor may be obtained by forcing in a stable manner to a low logic level the drive input nodes InU, InV and InW, while the enable inputs EnU, EnV and EnW are simultaneously driven to a low logic value and to a high logic value at the step-up frequency (>16 KHz). In this way, when the enable inputs are all in a high logic state, the output stage will be in a brake configuration with all the low side driver transistors turned on. When the enable inputs are at a low logic level, the output stage will be in a tristate configuration.
By forcing the inputs InU, InV and InW to a stable high logic level, an alternating condition of braking and tristate will always be obtained. But in this case, the brake configuration will be implemented by turning on all the high side driver transistors of the output stage.
FIG. 3 shows the timing diagram of the input and enable signals of the output bridge stage of the spindle motor to implement the step-up. The phase of braking is implemented by turning on the low side drivers.
The diagrams of FIGS. 4a and 4b, respectively show the phase of braking and the phase of tristating to better clarify the step-up function. In the representation of FIGS. 4a and 4b, reference is made to one of the six possible configurations of direction of the currents used in the rotating spindle motor during the phase of braking.
During the braking phase, all the low side MOSFETs are turned on, thus short circuiting the phase windings of the motor. In this configuration, the three BEMFs produce three currents of polarity and value that are a function of the angular position of the rotor. In the representations of FIGS. 4a and 4b, the current of the U phase is entering the motor, while the currents in the phase windings V and W exit the motor.
During the successive tristate phase, all the MOSFETs are turned off, and therefore, the three currents in the windings of the motor start to recirculate through the intrinsic diodes of the integrated structures of the power MOSFETs. More precisely, the outgoing currents from the windings V and W flow toward the V motor rail through the intrinsic diodes of the high side drivers, while the incoming current of the winding U comes from the ground rail flowing through the intrinsic diode of the low side driver. Thus, the recirculation loop of the three currents of the windings of the motor closes through the hold capacitor C3 connected between the Vmotor rail and the ground rail.
Every time the configuration of the output bridge stage switches from the “brake” configuration to the “tristate” configuration, the recirculating motor current charges the capacitor C3. This causes the voltage on the Vmotor node to be incremented.
The intrinsic diodes that are created in the integrated structures of the MOSFETs of the output bridge stage have a sensible conduction resistance that causes a non-negligible reduction of the voltage at which the hold capacitor C3 is charged during recirculation phases of the motor windings currents, i.e., when all the MOSFETs of the output bridge stage of the spindle motor are turned off (tristated).